The operation of conventional vortex sensors, as is well known, is based on the utilization of periodic pressure fluctuations in a Karman vortex street. Such a vortex street is formed when a fluid in a measuring tube flows against an obstruction, particularly a bluff body. From the downstream side of the body, vortices are shed, which form the vortex street. The frequency of vortex shedding is proportional to the volumetric flow rate of the fluid.
A conventional vortex sensor comprises the aforementioned bluff body and a measuring tube of predetermined length through which the fluid to be measured flows during operation. The measuring tube has an axis, an internal surface, an inlet end, an outlet end, a bore size corresponding to a nominal bore ordered by a customer, and a wall thickness suitable for a permissible pressure.
The bluff body has a horizontal cross-sectional area with a geometrical shape that can be selected by the manufacturer. It further has a first end and a second end which are connected with the wall of the measuring tube along a first fixing zone and a second fixing zone, respectively. The bluff body has a surface facing fluid flow and having a first and a second vortex-shedding edge, and is commonly disposed along a diameter of the measuring tube. The bluff body may have further vortex-shedding edges, particularly a third and a fourth vortex shedding edge.
A sensing element is fitted in the bluff body or is mounted downstream of the bluff body on the internal or external surface of the wall of the measuring tube or in this wall. The electric signals generated by the sensing element are processed by evaluation electronics and indicated and/or passed on to further electronics in the usual manner.
A characteristic feature of bluff bodies is that they have a surface facing fluid flow, at which the fluid is "dammed up". On the downstream side, the bluff bodies taper so as to obtain at least the first and second vortex-shedding edges and favor the vortex shedding.
The pressure variations associated with the vortices are converted into electrically processable signals by means of the sensing element mounted in or downstream of the bluff body, which may be a capacitive, inductive, or piezoelectric device, but also an ultrasonic transducer, for example. The frequency of these signals is directly proportional to the volumetric flow rate in the measuring tube.
Due to variations in the geometrical dimensions of produced vortex sensors, however, each of these devices must be calibrated individually, i.e., each device is measured in a calibration facility using a standard fluid, generally water. For this calibration measurement, the term "wet calibration" has come into use.
This is usually done by presetting several precisely known flow-rate values ("calibration values") by means of the calibration facility and registering the associated values indicated by the respective vortex sensor via the associated evaluation electronics. The deviation of the registered values from the precise values yields a calibration factor characteristic of the respective flow sensor.
The calibration factor is used, inter alia, to adjust a variable-gain amplifier stage in the evaluation electronics of the respective vortex sensor so that the indicated flow-rate values of all produced vortex sensors are equal to one another and to the above-defined calibration values.
The wet calibration described is complicated, time-consuming, and expensive. In the literautre, cf. "Bulletin of NRLM", Vol. 45, 1996, pages 174 to 179, a concept is described for dry-calibrating the produced vortex sensors based on determined physical dimensions and an experimentally optimized geometry of the individual bluff bodies. This optimization consists of determining that bluff body geometry for which the dependence of the Strouhal number on the Reynolds number is as linear as possible.
However, this method of dry calibration is not accurate enough since manufacturing tolerances of the bluff bodies and of other parts of the flow sensors are not taken into account. Furthermore, this method does not allow the use of bluff body shapes that may be necessary for other reasons. Moreover, present-day accuracy requirements, which are of the order of 0.75% of the measured value, cannot be met with the prior-art method.
It is therefore an object of the invention to provide a method of dry calibration which is much more accurate than the prior-art method.
To attain this object, the invention provides a method of dry-calibrating vortex sensors each comprising:
a measuring tube of predetermined length having a lumen PA1 a bluff body PA1 a sensing element PA1 producing, by means of a high-resolution electronic camera located on the axis in front of the measuring tube, in the direction of fluid flow, a digitized, two-dimensional overall image of the internal surface of the measuring tube in the area of the bluff body, the bluff body, the two fixing zones, and the contour line of the inlet end; PA1 dividing the overall image into a first, a second, and a third partial image, PA1 extracting from the first partial image PA1 extracting from the second partial image PA1 extracting from the third partial image PA1 forming from the first and second edge information PA1 forming from the distance information PA1 forming first cross-correlation information from the first shape information and from first ideal information characteristic of the ideal shape of the first fixing zone, and PA1 forming second cross-correlation information from the second shape information and from second ideal information characteristic of the ideal shape of the second fixing zone; and PA1 processing in a neural network PA1 together with respective standard information corresponding to said information and derived from a plurality of wet calibrations into PA1 forming from the distance information standard deviation information relating to all distances between the vortex-shedding edges of the bluff body; PA1 forming by means of a multiple comparator PA1 processing the calibration factor information, PA1 the dimension information, PA1 the contour reference information, PA1 the standard deviation reference information, PA1 the first roughness reference information, PA1 the second roughness reference information, PA1 the first cross-correlation reference information, PA1 the second cross-correlation reference information, PA1 the first surface defect reference information, PA1 the second surface defect reference information, PA1 the third surface defect reference information, and PA1 the fourth surface defect reference information into quality information and/or quality factor information. PA1 the bluff body has, in addition to the first and second vortex-shedding edges, a third and a fourth vortex-shedding edge; PA1 from the distance information, PA1 is formed; and PA1 in the neural network, the third and fourth roughness information is processed together with corresponding standard information derived from a plurality of wet calibrations into the calibration factor information and/or the distance information.
through which a fluid whose volumetric flow rate is to be measured flows during operation, and PA2 which has an axis, PA2 an internal surface, PA2 an inlet end, which forms a contour line with the lumen, PA2 an outlet end, PA2 a bore size corresponding to a nominal bore, and PA2 a wall thickness suitable for a permissible pressure of the fluid; PA2 which has a cross-sectional area with a geometrical shape selectable by the manufacturer, PA2 which has a first end connected with the wall of the measuring tube along a first fixing zone and PA2 a second end connected with the wall of the measuring tube along a second fixing zone, PA2 which has a surface facing fluid flow and having a first and a second vortex-shedding edge, and PA2 which is disposed along a diameter of the measuring tube; and PA2 which is fitted in the bluff body or PA2 which is mounted downstream of the bluff body on the internal or external surface of the wall of the measuring tube or in said wall, PA2 the first partial image containing virtually only information about the inlet end and the internal surface, PA2 the second partial image containing virtually only information about the bluff body without the fixing zones, and PA2 the third partial image containing virtually only information about the fixing zones; PA2 contour information about the contour line and PA2 first surface defect information relating to the internal surface of the measuring tube; PA2 first edge information about the first vortex-shedding edge of the bluff body, PA2 second edge information about the second vortex-shedding edge of the bluff body, and PA2 second surface defect information relating to the surface of the bluff body facing fluid flow; PA2 first shape information about the first fixing zone of the bluff body, PA2 second shape information about the second fixing zone of the bluff body, PA2 third surface defect information relating to the surface of the first fixing zone, and PA2 fourth surface defect information relating to the surface of the second fixing zone; PA2 distance information and PA2 angle information relating to the deviation of the vortex-shedding edges of the bluff body from parallelism; PA2 first roughness information relating to the not exactly straight course of the first vortex-shedding edge, PA2 second roughness information relating to the not exactly straight course of the second vortex-shedding edge, PA2 mean-value information for all distances between the vortex-shedding edges along the bluff body, and PA2 weighting information using a weighting function characteristic of predetermined flow profiles of the fluid; PA2 the contour information, PA2 the first and second roughness information, PA2 the mean-value information, PA2 the weighting information, PA2 the angle information, PA2 the first, second, third, and fourth surface defect information, and PA2 the first and second cross-correlation information PA2 calibration factor information and/or PA2 dimension information about the geometrical dimensions of the calibrated vortex sensor. PA2 contour reference information from the contour information and a contour limit value to be predetermined therefor, PA2 first roughness reference information from the first roughness information and a first roughness limit value to be predetermined therefor, PA2 second roughness reference information from the second roughness information and a second roughness limit value to be predetermined therefor, PA2 standard deviation reference information from the standard deviation information and a standard deviation limit value to be predetermined therefor, PA2 first cross-correlation reference information from the first cross-correlation information and a first cross-correlation limit value to be predetermined therefor, PA2 second cross-correlation reference information from the second cross-correlation information and a second cross-correlation limit value to be predetermined therefor, PA2 first surface defect reference information from the first surface defect information and a first surface defect limit value to be predetermined therefor, PA2 second surface defect reference information from the second surface defect information and a second surface defect limit value to be predetermined therefor, PA2 third surface defect reference information from the third surface defect information and a third surface defect limit value to be predetermined therefor, and PA2 fourth surface defect reference information from the fourth surface defect information and a fourth surface defect limit value to be predetermined therefor; and PA2 third roughness information relating to the not exactly straight course of the third vortex-shedding edge and PA2 fourth roughness information relating to the not exactly straight course of the fourth vortex-shedding edge
said method comprising the steps of:
A first preferred embodiment of the invention comprises the steps of:
In a second preferred embodiment of the invention, which can also be used with the first preferred embodiment,
One advantage of the invention is that the accuracy of the calibration is very good, namely of the desired order of 0.75% of the measured value.